Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

The experiment, done in Africa, mainly showed that the drug Viread is safe when used for prevention. Fewer people given the drug caught the AIDS virus than those given dummy pills, but so few in either group became infected that valid comparisons cannot be made, scientists said.

Still, “it’s incredibly encouraging,” said Dr. Helene Gayle, president of the antipoverty group CARE and co-chair of the International AIDS Conference in Toronto, where the results were released on Saturday.

If future studies show effectiveness, the drug “would be an incredibly important new prevention tool that we should make available as soon as possible,” she said.

A vaccine is considered the best hope for stopping the spread of AIDS, but scientists have not been able to make one that prevents infection. Condoms and counseling have not been enough — the virus spreads to 5 million more people worldwide each year.

The new approach involves Viread (known generically as tenofovir), a drug already used to treat AIDS. Animal research suggests that taking it before being exposed to HIV, through drugs or sex, could help prevent infection.

A study by Family Health International, funded by the Bill & Melinda Gates Foundation, tested it on women in Africa at high risk because of multiple sex partners (many were prostitutes). None had HIV at the start of the study. They were randomly assigned to get either daily Viread or dummy pills and all were counseled and given condoms.

No safety problems emerged — an important first step, said Leigh Peterson, project manager for Family Health. After an average of six months, only two HIV cases developed among the 427 women on Viread, compared with six infections among the 432 given the fake drug.

“We really would be irresponsible to draw conclusions at this time,” because those are too few cases to make judgments on, said Dr. Ward Cates of Family Health. “But it does underscore the importance of moving forward very quickly now on the other studies on the drawing board.”

Ethical concernsThe federal Centers for Disease Control and Prevention has studies under way in drug users in Thailand, heterosexual men and women in Botswana, and gay men in Atlanta and San Francisco. Plans are proceeding to add a third U.S. city, said Dr. Lynn Paxton, project leader at CDC.

The Thai results may be ready late next year and should reveal whether the strategy works. The Botswana study recently was changed to add a second drug, FTC, sold with tenofovir as Truvada by Gilead Sciences Inc. of Foster City, Calif.

However, the Africa study shows the ethical perils in such research. It originally was to include 1,200 women — 400 each in Ghana, Cameroon and Nigeria — but Nigeria and Cameroon ended participation prematurely over concerns about whether and for how long women in the study who became infected would receive treatment for AIDS.

Family Health worked to formalize contracts with local providers for testing and treatment as appropriate so the study could go forward on a limited scope, Peterson said.

And the study itself had benefits — half as many infections occurred than had been expected in the placebo group because the women cut back on the number of sex partners and condoms were used more often.

Some researchers have been worried that the opposite might happen if people believed a pill was protecting them.

“It’s good that they didn’t see an increase in risk” behaviors, said Dr. Susan Buchbinder of the San Francisco Department of Public Health, who is leading the CDC study in that city.

“We know that from all the trials we have done that even in the placebo group, risk is going to go down,” the CDC’s Paxton said.

Another study, funded by the National Institutes of Health, is expected to start later this year in Peru. It will test Truvada in 1,400 gay men in Peru. Family Health is working on plans for a new Truvada study as well.